This proposed project is a continuation and extension of studies on cell kinetics in experimental tumors of epithelial and mesenchymal origin, and of different growth rates. The kinetics of morphologically defined parenchymal cells of the proliferating and nonproliferating pool will be studied in tumors treated with a single dose and fractionated doses of low L.E.T. irradiation. 1) Specifically, studies will be concerned with the kinetic behavior of apparently "nonproliferating" cells, i.e., cells slowly traversing G1 phase or arrested in Go phase; namely, with their ability to re-enter the proliferating pool, with their presumed clonogenic potential, and with their participation in regeneration of irradiated tumors. 2) The capacity of nonproliferating cells to repair radiation damage will be compared with the repair potential of proliferating cells. 3) The relationship between nonproliferating and hypoxic cells will be studied under various experimental conditions. 4) The structural and ultrastructural composition of regenerating tumors will be studied in relationship to nonproliferating cells, re-stimulated to proliferate. BIBDIOGRAPHIC REFERENCES: Potmesil, M., and Goldfeder, A. (1977a) Identification and kinetics of Gl phase-confined cells in experimental mammary carcinomas. Cancer Res., 37:857. Potmesil, M., LoBue, J., and Goldfeder, A. (1977b). Stem cells, nonproliferating cells and their kinetics in normal and neoplastic tissues. In: F.F. Becker (ed.), Cancer - A Comprehensive Treatise, Vol. 6, in press.